1. Field of the Invention
The invention relates to voice coil motors, and more particularly to voice coil motors with reduced size, manufacturing costs, and electrical power consumption and enhanced positioning precision.
2. Description of the Related Art
Cameras disposed in cellular phones have been developed to provide high definition quality and reduced power consumption, manufacturing costs, and size. As such, actuators disposed in the cameras, for moving lens modules thereof, are critical.
Automatic displacement driving devices applied in lens modules may employ drivers providing rotational power with a rotational axis thereof paralleling an optical axis of a lens module or drivers providing movement power with a moving direction thereof paralleling the optical axis of the lens module.
An example of a driver providing rotational power with a rotational axis thereof paralleling an optical axis of a lens module is a stepping motor. The driver requires additional transmission-conversion mechanisms to enable the lens module to move along an optical axis. When the lens module arrives at a final position, no electricity is required to maintain the lens module therein. However, the driver has many components. Thus, the structure of the driver is complicated, and the size thereof cannot be reduced.
An example of a driver providing movement power with a moving direction thereof paralleling an optical axis of a lens module is a voice coil motor, a piezoelectric actuator, or a liquid lens actuator. The driver directly adjusts the position of the lens module. Compared with the driver providing rotational power, this driver has fewer components and is small. Nevertheless, there is a need to further reduce the size and enhance precision of this driver.
Taiwan Patent Publication No. 200525859 discloses a voice coil motor with two opposite non-annular magnets and multiple yoke sets. The non-annular magnets and yoke sets form a movable magnetic assembly. Part of the magnetic lines is output from the surface of the non-annular magnets and is transmitted to a magnetic-permeable shaft via the yoke sets, generating radial attraction (radial pre-compression force) between the movable magnetic assembly and the magnetic-permeable shaft. The radial attraction enables the movable magnetic assembly to slide with respect to the magnetic-permeable shaft in a smooth manner. Moreover, in the voice coil motor, generation of a voice coil force between a coil and the non-annular magnets utilizes part of the magnetic lines with the same direction and route.
Referring to FIG. 1A and FIG. 1B, Japan Patent Publication No. 2005-128405 discloses a conventional lens driving device 1. An upper spring 9 and a lower spring 11 enable precise movement of a lens module 20 and reduce friction during movement thereof. The upper spring 9 and lower spring 11 may be regarded as extensions of the coil 15, serving as conductors at two ends thereof. Specifically, in the lens driving device 1, the upper spring 9 and lower spring 11 can provide axial pre-compression force to the lens module 20. Thus, the lens module 20 can be easily positioned in a specific position when the coil 15 is energized by application of a current. Nevertheless, as the lens driving device 1 must comprise a yoke 3 providing magnetic-permeable functions, the size and manufacturing costs thereof cannot be reduced.
Taiwan Patent No. M305361 discloses a voice coil motor having a permanent magnet with a two-pole magnetization direction. Namely, the magnetization direction of the permanent magnet is perpendicular to a moving direction thereof. The permanent magnet with the two-pole magnetization direction can enhance utilization of magnetic flux. Nevertheless, if the shape of the permanent magnet is not a cuboid, this has further resulting in poor production of the permanent magnet due to the difficult magnetization. The cost and the size thereof cannot be reduced.
Japan Patent Publication No. 2003-207708, Japan Patent Publication No. 2006-220776, and U.S. Pat. No. 5,220,461 disclose voice coil motors with a magnetization direction of a permanent magnet perpendicular to a moving direction thereof (i.e. the permanent magnet is radially magnetized). To enhance utilization of magnetic flux from the permanent magnet, magnetic-permeable yokes are disposed on the radial lateral of the permanent magnet. However, reduction of the size of the voice coil motors is not easily achieved. Moreover, in the voice coil motors, to pass a lens module through a coil, the coil must provide a large hollow portion. Accordingly, the winding turns of the coil providing the large hollow portion are limited, adversely affecting enhancement of a voice coil force. Furthermore, the overall strength of the coil providing the large hollow portion is insufficient, causing deformation during assembly thereof, and further resulting in poor production of the voice coil motors.
Taiwan Patent Publication No. 200525859 and Taiwan Patent No. 176799 disclose voice coil motors with a magnetization direction of a permanent magnet parallel to a moving direction thereof (namely, the permanent magnet is axially magnetized). In the voice coil motors, a coil is disposed between multiple permanent magnets and magnetic-permeable yokes and two gaps exist between a movable part and a fixed part. Specifically, one gap exists between the permanent magnets and the coil while the other exists between the magnetic-permeable yokes and the coil. Due to the two gaps and magnetic-permeable yokes, the size thereof cannot be reduced. Moreover, as guide bars of the voice coil motors are composed of magnetic-permeable material, magnetic attraction is generated between the guide bars and the permanent magnets, such that a starting current required to overcome sliding friction is increased. Additionally, considerations exist concerning whether there is limitation for the winding turns of the coil and whether disposition of a sensor matches the outer circumference of the coil.
Hence, there is a need for voice coil motors with reduced size, manufacturing costs, and electrical power consumption and enhanced positioning precision.